Current-carrying windings are used to generate magnetic fields. In particular, they can be used for electromagnetic energy conversion. For example, electric energy can be converted to mechanical energy by means of a winding (electromotive principle). If the winding is employed in a transformer, the winding serves to convert electric energy into electric energy.
The amount of electromagnetically converted power is governed by the product of the voltage across the winding and the current flowing through the winding. For motor operation, the voltage and current are generally subject to certain constraints. For example, the voltage of an integral starter/generator must not exceed the vehicle electrical system voltage of e.g. 42 volts.
The ratio of voltage to current can be varied by means of the voltage-holding turns count of the winding. This enables the operating range of the voltage and current to be adapted to suit requirements with the electromagnetically converted power remaining unchanged. It is therefore desirable to be able to finely adjust the voltage-holding turns count.
In the case of a rotating field winding laid in slots, as frequently used in electric motors, the voltage-holding turns count of a phase of the winding is given by the following formula:   w  =            p      ×      q      ×              z        N              a  where p is the number of pole pairs of the winding, q the number of slots per pole per phase, zN the number of conductors per slot and a the number of parallel paths per phase of the winding. A phase is part of the winding and defined in that all its paths are connected in parallel and the same control voltage is applied to them.
The parameters p, q, zN and a will be explained in the following with reference to two examples of the prior art.
FIG. 1a shows a cross-section through a motor stator with a first winding W according to the prior art. The stator S has twenty-four slots 1, . . . , 24. The winding W is implemented as a wave winding and consists of three phases each with a single path W1, W2, W3. For this example, therefore, a=1. The path W1 of a first phase of the winding W is runs along eight of the slots 1, 4, 7, 10, 13, 16, 19, 22. At the outer sides of the stator S are located sections of the winding W, the so-called end connectors, which interconnect the sections of the winding W laid in the slots.
FIG. 1b schematically illustrates the path W1 of the first phase of the winding W from FIG. 1a. The vertical sections of the path W1 are laid in the slots, whereas the horizontal sections of the path W1 constitute the end connectors. A vertical and a horizontal section of the path W1 form a magnetic pole P. The winding W therefore has eight poles P and consequently four pole pairs. For this example, therefore, p=4. As each pole obviously only has one slot, we have q=1. Likewise obviously, only one conductor is disposed in each slot involved, so that zN=1. The voltage-holding turns count of the first winding W is consequently   w  =                    4        ×        1        ×        1            1        =    4.  
FIG. 2 shows a schematic view of a phase of a second winding according to the prior art. Each phase of the second winding has a single path W1′, so that a=1. The path W1′ is wound twice around the circumference of the stator. The path W1′ therefore has two sub-sections T1, T2 each winding the stator once as a wave winding. In addition, the second winding has four poles and therefore two pole pairs, each pole being assigned two slots, i.e. each pole is formed by two slot coils. Thus the first pole is assigned the slots 1 and 2. Consequently q=2. As in the first example, zN=1. The voltage-holding turns count of the second winding is therefore   w  =                    2        ×        2        ×        1            1        =    4  
The number of pole pairs and the number of slots per pole per phase are determined according to the application or limited by external constraints. Thus, for example, in the case of the starter/generator, the mounting space generally requires a high pole pair count. In the case of asynchronous machines, for example, a value of at least q=3 is selected where possible for the number of slots per pole per phase in order to minimize the harmonic field scattering and thereby improve operating performance. Nor can the number of parallel paths be randomly selected in order to obtain a required voltage-holding turns count. As explained e.g. in “Asynchronmaschinen” (Asynchronous Machines), H. Jordan et al, pp. 105-106, the number of poles 2p must be an integral multiple of the number of paths, as the paths each wind different sub-areas of the magnetic circuit and these sub-areas must be of equal length (cf. also “Lehrbuch der Wicklungen elektrischer Maschinen” (Manual of Electric Machine Windings), R. Richter, pp.105-106). Moreover, it is generally disadvantageous to provide more than one path, as lossy circulating currents between the paths are produced in the event of asymmetries in the motor (e.g. rotor eccentricities) if there are a plurality of paths. If a risk of asymmetries exists, a single path is generally provided per phase, i.e. a=1.
Essentially, therefore, the voltage-holding turns count must only be influenced by the number of conductors per slot. However, a change of only one in the number of conductors per slot means that the voltage-holding turns count is changed by a factor of             p      ×      q        a    .Fine adjustment of the voltage-holding turns count and therefore of the ratio of voltage to current by the winding cannot be achieved in this way. This applies particularly to cases in which the number of conductors per slot must be low because of the required relationship between voltage and current.